1. Field of the Invention
The present invention relates to an image forming system, and more particularly, to a method and an apparatus to control a high voltage output while reducing loads on a main processor in an engine controlling module by simplifying a control signal line.
2. Description of the Related Art
In an image forming system using an electro-photographic method, when light corresponding to image information is scanned by an exposure device onto a photosensitive medium which is thus charged to a predetermined electric potential, an electrostatic latent image is formed on the photosensitive medium. Then, a developer provides toner on the electrostatic latent image to form a toner image. In a color electro-photographic method, four developers, each of which contains a color toner such as cyan, magenta, yellow, and black, are required to form an image. The formed toner image is transferred to a sheet of paper directly from a photosensitive medium or by passing through an intermediate transfer medium. When the transferred toner image passes through a fuser, the toner image is fused on the paper by heat and pressure. The mono-color image or multiple color image is printed on the paper via the above processes.
A color image can be formed with, for example, a single-pass method, wherein four exposure units and four photosensitive media are used, or a multi-pass method, wherein one exposure unit and one photosensitive medium are used. In both methods, four color toners as described above are required. The printing speed in the single-pass method is the same for mono-color printing and multiple color printing, thus high speed color printing can be performed. In the multi-pass method, it takes at least four times longer to print a multiple color image as opposed to a mono-color image. However, a printing operation in the multi-pass method can be performed with an apparatus having a simpler structure than that used to perform a printing operation in the single-pass method.
In the above image forming system, a plurality of high voltages are required to charge, develop, transfer, clean, and paper adsorb. In the single-pass method, 20 pulse width modulation output ports including 4 ports to supply charging high voltage, 4 ports to supply developing high voltage, 4 ports to supply a first transfer high voltage, 4 ports to supply cleaning high voltage, and 2 ports to supply paper adsorbing high voltage, and 19 control ports are required. On the other hand, the multi-pass method requires 4 PWM output ports including a port to supply charging high voltage, one port to supply developing high voltage, one port to supply a first transfer high voltage, and one port to supply second transfer high voltage, and control ports corresponding to the PWM ports.
In the single-pass method, since more control ports for high voltage output are used, more control signal lines between a main processor including an engine controlling module and a high voltage output module are required. Additionally, more pins are necessary and a total length of a harness increases. Thus, the single-pass method has disadvantages such as high fabrication cost and high sensitivity to errors due to various noises. Also, since control signals of the high voltage output module are generated in the main processor of the engine controlling module, the main processor should operate the engine and control the high voltage output simultaneously. Thus, an expensive main processor operating at a higher speed is required.